The present invention relates to a V-shaped internal combustion engine, particularly, a V-shaped internal combustion engine having a balancer device for canceling a secondary vibromotive force thereof.
Conventionally, there have been proposed many engines each comprising a sub-chain for driving a balancer device, an oil pump, a water pump and the like in addition to a timing chain for connecting a valve cam on a cylinder head and a crankshaft so as to drive said valve cam (for instance, Japanese Unexamined Patent Publication No. Sho. 62-233423).
For instance, if a plane crank is adopted in a four-cycle V-shaped eight cylinder engine having a bank defining angle of 90 degrees (which is formed between the V-shaped banks of cylinders) in which plane crank axial centers of all crank pins are located on the same plane, operating cycles of two cylinder banks shift 180 degrees, and explosions take place in the respective cylinder banks in an alternate fashion. According to this construction, since explosions on one of the banks are timed at a regular interval, causing no exhaust interference, the plane crank configuration is advantageous in achieving a high output. On the other hand, in the V-shaped eight cylinder engine adopting the plane crank, secondary imbalance is generated by virtue of an inertia force generated in turn by the reciprocating mass of the engine. The direction of the inertia force so generated while the secondary imbalance is being generated becomes similar to that of an inertia force generated in a state in which cylinders of a conventional in-line four cylinder engine are made horizontal when the engine is viewed as a whole. The aforesaid imbalance can, therefore, be compensated for by adopting the theory of the secondary balancer for a conventional in-line four cylinder engine, and rotating in opposite directions to each other two balancer shafts disposed at symmetrical positions with respect to a horizontal plane acting as a center therebetween which bisects the bank defining angle and passes through the center of a crankshaft (refer to Japanese Unexamined Patent Publication No. Hei. 8-193648).
When trying to provide the aforesaid balancer in the V-shaped eighth cylinder engine adopting the plane crank, it is practical to provide the same at a lower portion of one of the cylinder heads so as to be driven by the sub-chain, as shown in the above Japanese Unexamined Patent Publication No. Hei.8-193648.
On the other hand, since with a V-shaped engine having a bank defining angle of 90 degrees two cylinder heads are spaced away form each other relatively wide, it is the normal practice that separate endless power transmission timing belts are provided between the crankshaft and the respective cylinder heads individually therefor. In this case, it is natural that the balancer is disposed such that it does not interfere with an endless power transmission timing belt. In addition, however, in a case where the balancer is driven by means of a chain, the driver chain also has to be disposed such that it does not interfere with the endless power transmission timing belt.
This requires the triple provision of pulleys or sprockets on the crankshaft and due to this the engine tends to be expanded in the axial direction of the crankshaft. This is a first problem in the conventional technique.
However, it is the normal practice that a guide for the valve cam driving timing chain and a guide for the sub-chain are provided separately. In this case, the chains are spaced away from each other so that they do not interfere with each other, and the chain guides have to be enlarged unnecessarily in order to secure support portions for the chain guides, these eventually leading to a problem of the engine being made larger in size and heavier in weight. This is a second problem in the conventional technique.